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Dietary Fats, Linolenic Acid and the Risk of Prostate Cancer (June 2004)
"A high content of
dietary fat increases the incidence of prostate cancer" sounds like a
simple declarative statement of scientific fact, but there are few
things in nutritional science that are completely straightforward.
Flesher et al.. reviewed this complex subject in the February issue of
the Journal of Urology and largely supported the hypothesis that dietary
fats are likely a "late stage environmental promoter...responsible for
converting microscopic foci of latent cancer into larger volumes of more
aggressive cancer". Studies of saturated fat intake showed a positive
association of high levels of fat intake with advanced prostate cancer.
In their review of the 33 analyses of the relationship of dietary fat
and prostate cancer, 8 found a statistically significant relationship
and many showed a trend. The 1993 report from the Health Professionals
Follow-Up Study "found that saturated fat from animal sources such as
red meat and dairy products were linked to a 2.6-fold increased in
prostate cancer"(Harvard Men's Health Watch, Jan 2002). One drawback of
retrospective nutritional studies is the reliance on personal dietary
histories, which can be quite vague. And ultimate complexity in
nutritional analysis stems from the combined action of man's teeth and
stomach, the world's first and best Cuisinart, wherein every dietary
ingredient intermingles and affects of all the others. The article in
the Journal of Urology offers, four possible mechanisms that might
underlie the fat-prostate cancer association: 1) dietary fat tends to
increase androgen production; 2) pesticides, many of which are fat
soluble, may promote cancer; 3) the metabolism of fats produces cellular
oxidative stress, which induces DNA mutations; and 4) a high fat diet
contains a greater amount of the specific essential fatty acid, alpha-linolenic
acid, which is the subject of the following discussion.
Dietary fats
enter our metabolism as fatty acids, composed of chains of up to as many
as 24 carbon atoms containing from none (saturated) up to 6 double bonds
(polyunsaturated). A great deal of attention has been focused on alpha
linolenic acid (ALA), an omega-3 fatty acid (FA) having 18 carbons,
three double bonds, the last occurring on the third carbon from the last
in the chain, the "omega" carbon atom. ALA is one of two "essential"
fatty acids, essential because man cannot manufacture it from smaller
building blocks, in contrast to the non-essential FAs. ALA naturally
enters human's diet in quite small quantities principally from plant
sources such as beans, walnuts, and wheat germ; and as supplements from
oils, such as flaxseed (50% ALA), canola (11% ALA), and soybean (7%
ALA). Olive oil contains only 1% ALA. A key "take-home" point of this
discussion is that ALA is not the only "omega-3" FA. The
other important omega-3s are the so called "long[er] chain" FAs with 20
and 22 carbons, and these are found abundantly in marine oil supplements
and in oily, dark fleshed fish such as salmon, tuna, herring, mackerel
and bluefish. The other essential FA is linoleic acid, an omega-6 FA,
abundant in the human diet from meat and dairy products, and this FA has
been established as a promoter of prostate cancer. To put the dietary
content of the two essential FAs in comparative perspective, a Norwegian
nutritional study determined blood levels of FAs in 423 men and
identified their dietary derivation. They found that 625 mg/l of FAs
came from saturated fat, 422 mg/l from omega-6 polyunsaturated acids (PUFAs)-
largely linoleic, and 100 mg/l from omega-3 PUFAs, of which only 2 mg/l
were derived from ALA. The remaining content of omega-3s came from the
longer chain FAs derived from fish.
Why the interest
in alpha-linolenic acid? The attention is merited. The Harvard Men's
Health Watch, Jan 2002, cited the Lyon Dietary Health Study, which
studied a "Mediterranean" type diet enriched with canola margarine, and
showed that "Over a four-year period, the high ALA diet produced a 72%
reduction in heart attacks and cardiac deaths and 56% lower risk of
dying from any cause (including prostate cancer)". However, the
Physicians' Health Study found that "Men with moderately high ALA blood
levels were 3.4 times as likely to develop prostate cancer than men with
the lowest levels" (Harvard Men's Health Watch).
So the question
has emerged "Can what is good for the heart be bad for the prostate?"
And the answer is ... possibly, or it's equivocal, or "more study is
needed." This issue was nicely reviewed in the Journal of Urology, April
2004, in "Alpha-linolenic acid and the risk of prostate cancer. What is
the evidence?" Many other studies have addressed this puzzling issue.
The review found that the many studies that implicated ALA in increasing
prostate cancer risk were counterbalanced by others wherein ALA had
either no effect on prostate risk and prostate cancer aggressiveness, or
even showed a benefit. A large 1999 Netherlands analysis of 58,278 men
over 6 years found ALA associated with a decreased risk of prostate
cancer.
To link this issue
with the first article in this PCa Commentary, omega-3 PUFAs can be
metabolized by both lipoxygenase (such as 5-LOX) and cycloozygenase
(COX) enzymes, and the products have been found to be both
procarcinogenic and anticarcinogenic! The review J UROL article, after
contrasting evidence for and against ALA's role in increasing the risk
of prostate cancer concluded, more research is needed before
recommendations can be made regarding the association of this essential
nutrient [ALA] and prostate cancer.
So what do we do -
tonight, at the dinner table? After researching this issue, I would
suggest reducing dietary animal fats and, for now, "hold the canola".
And I concur with the recommendation of the Harvard Men's Health Watch:
"Rely on fish for your omega-3s".
Bottom Line:
The old adage holds: "We are what we eat." and what we eat affects the
diseases we are prone to.
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